1. Field of the Invention
The present invention relates to an ink jet printer for printing on a printing medium with ink droplets ejected from nozzles.
2. Description of the Related Art
FIG. 1 shows a conventional ink jet head which ejects ink droplets from nozzles utilizing displacements of piezoelectric elements. FIG. 2 shows a driving system of the head shown in FIG. 1. As shown in FIG. 1, a head unit 92 having an ink cartridge 91 is detachably mounted on a carriage 93. A driver IC substrate 94 on which a driving circuit 60 (see FIG. 2 is mounted is attached to the bottom portion of the head unit 92. The driver IC substrate 94 is connected to a power source circuit 40 and a control circuit 70 shown in FIG. 2 via a dimple flexible print circuit (FPC) 95 mounted on the upper surface of the carriage 93. As shown in FIG. 3, clock signal (CLK) 62, data signal (DATA) 63, strobe signal (STB) 65 and drive signal 100 are output to the driving circuit 60 from the control circuit 70.
A drive current V1 (see FIG. 3) flows in a drive power source line 74 connecting the power source circuit 40 and the driving circuit 60 shown in FIG. 2. The drive current V1 has an acute peak caused by rising, i.e., transition time, of the driving signal 100 output from the control circuit 70. Due to the acutely peaked drive current V1, the voltage of the power source circuit 40 instantaneously drops by virtue of the resistive component of the dimple FPC 95. The voltage drop of the power source circuit 40 does not sufficiently deform the piezoelectric element, resulting in weak ejection of the ink droplets.
In order to decrease the voltage drop, it has been proposed to connect a smoothing capacitor (or bypass capacitor) 76 between the drive power source line 74 in the vicinity of the driving circuit 60 and a ground line 75 as shown in FIG. 2. When replacement or repair of the head unit 92 are necessary due to malfunction, the power supply from the power source circuit 40 is interrupted and thereafter replacement of the head unit 92 is performed.
However, even if the power supply from the power source circuit 40 is interrupted, electric charges stored in the smoothing capacitor 76 cause a latchup current when a contact in a control system is opened while maintaining a high voltage contact in a closed condition. The latchup current may damage the driving circuit 60.